1. Field of the Invention
This invention relates to an actuator and a driving apparatus of the actuator, and especially relates to a truss type actuator or an impact type actuator for moving a driven object by utilizing a displacement generated in a piezoelectric device when an alternating driving voltage is applied thereto.
2. Description of the Related Art
A piezoelectric device shows piezoelectric effect for expanding or contracting a displacement substantially in proportion to a level of a voltage when the voltage is applied in a direction parallel to the polarization direction thereof. Thus, a piezoelectric device is conventionally proposed to be used as an actuator for supplying a driving force to a driven object. The piezoelectric device used in the piezoelectric actuator is polarized in unidirectional. Electrodes are provided on both end faces of the piezoelectric device in the polarization direction thereof and output terminals of a driving circuit are connected thereto, so that driving signal from the driving circuit can be applied between the electrodes.
In an impact type piezoelectric actuator, a friction member, which contacts a driven object, is provided to contact an end face of the piezoelectric device in a direction parallel to the direction of the displacement (expansion or contraction) of the piezoelectric device. An impact voltage signal rapidly rising up is applied to the piezoelectric device for quickly expanding (or contracting) the piezoelectric device, so that one of the friction member and the driven object is relatively moved in a direction. Subsequently, another voltage signal slowly falling down is applied to the piezoelectric device for slowly contracting (or expanding) the piezoelectric device, so that the friction member and the driven object are moved in the opposite direction at the same time. Thus, the driven object is moved in a predetermined direction.
In a truss type piezoelectric actuator, two rod-shaped piezoelectric devices are provided for crossing at a predetermined angle, for example, at right angle and a driving member contacting a driven object is provided at a cross point of the piezoelectric devices. Two alternating driving voltages having a phase difference between them are respectively applied to the piezoelectric devices for moving the driving member along an elliptic trail. Thus, the driven object, which is intermittently moved with the driving member, is rotated or linearly moved in a predetermined direction (see, for example, U.S. Pat. No. 4,613,782).
In such the piezoelectric actuator, the displacement (quantity of the expansion and the contraction) can be increased by increasing the level of the voltage applied to the piezoelectric devices. However, when the piezoelectric actuator is used, for example, in a handy type equipment such as a camera, it is difficult to generate a high voltage driving signals for driving the piezoelectric devices, since electric energy of a battery is consumed by electric devices such as a motor for winding a film or for adjusting a position of a taking lens, an electromagnet for controlling shutter speed or for controlling a size of an aperture diaphragm, and so on. Furthermore, a DC/DC converter, a smoothing capacitor, and so on are newly necessary or become larger than the conventional ones for increasing the voltage of the driving signals of the piezoelectric devices. Thus, a configuration of a driving signal generating circuit becomes more complex and larger than the conventional one, so that it is difficult to downsize the equipment using the piezoelectric actuator and to reduce the cost of the driving circuit or the equipment.
For information, in the art of audio equipment, it is conventionally known that an input signal of one of two amplifiers is inverted and output terminals of both of two amplifiers are respectively connected to coils of speakers (Bridge Tied Load connection) for increasing output power of the speakers. However, such the conventional method merely inverts the audio signal and the object to be applied is the electromagnet which has no relation with respect to the polarization. Thus, it is impossible to apply the conventional method to the piezoelectric device for increasing the output power thereof, since it is necessary to regard the polarization in the piezoelectric device.
An object of the present invention is to provide an actuator and a driving apparatus thereof, in which voltage of the driving signals of the actuator is maintained in a low level with regard to inversion of the polarization, but the displacement of the actuator can be increased double. Furthermore, the apparatus using the actuator and the driving apparatus can be downsized and be inexpensive.
An actuator driven by a driving apparatus comprises a piezoelectric device serving as a driving source when a driving signal is applied in a polarization direction thereof. A driving apparatus in accordance with an aspect of the present invention comprises: a waveform generator for generating a signal varying corresponding to the passage of time; a first driver for generating a first voltage signal having a maximum voltage smaller than a voltage of inversion of polarization of the piezoelectric device by using the signal from the waveform generator, and for applying the first voltage signal to the piezoelectric device in the polarization direction; and a second driver for generating a second voltage signal having a maximum voltage smaller than the voltage of inversion of polarization of the piezoelectric device and the same polarity as that of the first driving signal by using the signal from the waveform generator, and for applying the second voltage signal to the piezoelectric device in a direction opposite to the polarization direction.
By such a configuration, the piezoelectric device is substantially driven by a driving signal amplified double of the conventional driving signal, so that the displacement of the piezoelectric device becomes substantially double in comparison with the conventional one. However, the voltage of the driving signal can be much smaller than the voltage of the inversion of polarization of the piezoelectric material. The present invention can be applied to both of a truss type actuator and an impact type actuator.